Aluminum borate ceramic matrix composite

ABSTRACT

Disclosed is a high strength aluminum borate ceramic matrix composite product and a method of making the same which comprises providing a slurry of aluminum oxide, boron oxide and organic or aqueous binder, the aluminum oxide and boron oxide capable of reacting to form aluminum borate. The slurry is introduced to a body, e.g., web of ceramic fibers, to provide an infiltrated web. After removing liquid and organic binder from the infiltrated web, it is sintered to react the aluminum oxide and boron oxide to form a ceramic matrix composite comprised of aluminum borate and said web of ceramic fibers. The reaction causes a volume expansion which aids in filling or removal of pores or voids resulting from the removal of organic binder.

INTRODUCTION

This invention relates to ceramic composites and more particularly, itrelates to ceramic matrix composites.

The need for advanced materials in high performance engineering systemshas prompted consideration of ceramics, which offer many advantages overmetals and alloys. Ceramics provide high temperature capability,environmental stability and low density. However, the use of ceramics isseverely limited because of their extreme sensitivity to flaws andbrittle character. With the availability of continuous ceramic fibers,there has been a tremendous interest in the development of ceramicmatrix composite (CMC) possessing high temperature capability (T>1500°F.), noncatastrophic failure mechanisms and low density. For CMC tofunction well, the matrix, fibers and the interface have to becompatible. Also, for economic and practical reasons, the processingroute should preferably be simple, inexpensive and capable ofmanufacturing large and complex shapes.

Ceramic matrix composed of lightweight ceramic materials formed fromaluminum oxide mixed with phases of other metal oxides are desirablefrom the standpoint of weight as well as chemical inertness. U.S. Pat.No. 4,698,319 describes a ceramic which comprises an interwoven mixtureof TiB₂ and Al₂ O₃ formed by reacting together a mixture of TiO₂, B₂ O₃and aluminum metal. This ceramic material has been found to possessexcellent chemical inertness properties even at elevated temperatures.

However, such ceramic materials or cermets are not necessarilycharacterized by light weight nor do they always possess high strengthqualities, particularly if they have been blended to optimize otherproperties such as chemical inertness and electrical conductivity.Further, monolithic ceramics can have very high strength and elasticmoduli, but they tend to be brittle.

The formulation of ceramic materials from oxides of aluminum and boronwould be expected to be somewhat lighter than aluminum oxide, dependingupon the amount of boron oxide used, because the density of aluminumoxide is about 3.9 and the density of boron oxide is about 2.46.Ceramics made from such oxides are known.

Sowman U.S. Pat. No. 3,795,524 describes the formation of transparentextrusions such as fibers of aluminum borate and aluminum borosilicatematerials from an aqueous solution or dispersion, e.g., an aqueoussolution of a boric acid-stabilized aluminum acetate, which isconcentrated into extrudable gels, subsequently dried, and then fired attemperatures up to 1000° C. to form fibers of transparent aluminumborate or aluminum borosilicate. The patentee states that low densityaluminum borate fibers may be formed in this manner having an Al₂ O₃ :B₂O₃ mole ratio of from 9:2 to 3:1.5. Sowman, however, cautions againstfiring at temperatures as high as 1200° C., stating that fibers fired atthis temperature are weak and fragile.

DeAngelis U.S. Pat. No. 4,540,475 discloses the formation of a multiplephase body containing phases of TiB₂, Al₂ O₃ and 9Al₂ O₃ 2B₂ O₃. Thebody was formed from a dry mixture of AlB₂, TiO₂ and Al₂ O₃ which waspressed at 1500 psi and then fired at 1500° C.

Baumann and Moore in an article entitled "Electric FurnaceBoroaluminate" in The Journal of the American Ceramic Society, Oct. 1,1942, Vol. 25, No. 14, disclose that boroaluminate has been produced asa crystalline material by electric furnace fusion. The crystal form isorthorhombic, and it appears to melt incongruently and is analogous inseveral ways to mullite.

Ray U.S. Pat. No. 4,774,210 discloses a sintering aid suitable for usein forming aluminum borate into a ceramic product, the sintering aidcomprised of an aluminum borate compound having a melting point lowerthan the sintering temperature of aluminum borate.

Ray U.S. Pat. No. 4,804,642 discloses an aluminum borate base ceramiccomposite comprised of a metal compound and a composition having theformula Al_(y) B_(x) O_(z).

Ray U.S. Pat. No. 4,804,646 discloses a high strength low density opaqueshaped aluminum borate product characterized by an MOR of at least45,000 psi and a density of approximately 2.9 g/cm³. The shaped aluminumborate may be formed by reacting an aluminum oxide with a boron oxide ata temperature of at least 800° C., grinding the reaction product,pressing the resulting particulate into a shaped form and sintering theshaped particulate at a temperature of from 800° to 1400° C. whilemaintaining the shaped article under a pressure of 2500 to 3500 psi.

U.S. Pat. No. 4,774,210 discloses that a lightweight aluminum boratematerial can be fabricated without pressure during sintering.

One of the problems in forming ceramic matrix composites is obtaining ahighly densified product. That is, whenever an organic binder is burnedout of the ceramic matrix composite, porosity results. Differentapproaches are used to fill the porosity to increase the density of thecomposite. One approach is chemical vapor infiltration. However, duringchemical vapor infiltration, material deposits on the surface of thecomposite, and holes or pores are closed, preventing furtherdensification. Thus, it can be seen that there is a great need for asystem which can substantially eliminate the porosity. The presentinvention solves this problem by providing a system wherein theformation of ceramic during sintering has a volume expansion which iseffective in reducing or eliminating porosity.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a high strength aluminumborate matrix composite product.

It is another object of this invention to provide a ceramic matrixcomposite which effectively reduces porosity during sintering.

It is yet another object of this invention to provide a high strengthaluminum borate matrix composite product formed by reacting together amixture of an aluminum oxide and a boron oxide in the presence of areinforcing ceramic fiber, for example.

In accordance with these objects there is provided a high strengthaluminum borate ceramic matrix composite product and a method of makingthe same which comprises providing a slurry of aluminum oxide, boronoxide and organic binder, the aluminum oxide and boron oxide beingprovided in proportions capable of reacting to form aluminum borate. Theslurry is introduced to a web or webs of ceramic fibers to provide aninfiltrated web. After removing liquid and organic binder from theinfiltrated web, the green composite is sintered to react the aluminumoxide and boron oxide to form a ceramic matrix composite comprised ofsaid web of ceramic fibers and aluminum borate. The reaction causes avolume expansion which aids in filling or removal of pores or voidsresulting from the removal of organic binder.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a micrograph of a fractured ceramic matrix composite inaccordance with the invention.

FIG. 2 is a micrograph of a fractured ceramic matrix composite at ahigher magnification in accordance with the invention.

DETAILED DESCRIPTION OF THE INVENTION

A high strength ceramic matrix composite is formed and is comprised of aweb or mat of fibers of silicon carbide, for example, and aluminumborate. The composite is formed by providing a slurry comprised ofpowders of aluminum oxide and boron oxide provided in an organic oraqueous liquid containing a binder. The amount of solids in the slurrycan range from 40 to 70 vol. %. The solids which comprised alumina andboria are preferably in the ratio of 9 to 2 (4.5). However, the ratio ofalumina to boria may range from 2 to 8. The slurry is introduced to aweb or cloth of suitable ceramic fibers and dispersed throughout theweb. Several layers of the infiltrated web may be provided as a bodyhaving increased thickness. The body or layers may be squeezed orpressed to remove excess liquid and thereafter dried and heated to burnoff organic material. The heated body is sintered so as to cause thealuminum oxide and boron oxide to react in situ to form an aluminumborate matrix thereby forming a ceramic matrix composite of ceramicfibers and said aluminum borate, preferably having a density of at least90%. Preferably, the molar ratios of Al₂ O₃ and B₂ O₃ are controlled toform Al₁₈ B₄ O₃₃ during reaction sintering. In its broader aspects, thealuminum borate may be represented by the formula Al_(x) B_(y) O_(z)wherein x is equal to 16-20, y is equal to 3-5 and z is equal to 30-36.

The aluminum oxide to be mixed with the boron oxide powder may compriseAlcoa A-16 super ground alumina having an average particle size of 0.4micrometer while the boron oxide may comprise Fisher A-76 B₂ O₃ powderor boria powder which should also have an average particle size of about-325 mesh (Tyler). The powder mixture may then be blended and milled,e.g., ball milled, prior to adding to the binder.

Other aluminum compounds, e.g., Al₂ O₃.3H₂ O, Al₂ O₃.H₂ O, AlCl₃.6H₂ O,AlCl₃, and Al(NO₃)₃.9H₂ O, may be used along with boric acid, e.g., H₃BO₃.

When the body is heated up to 1000° C., the major constituents are Al₂O₃ and 2Al₂ O₃.B₂ O₃ type phase. On further heating to above 1035° C.,there is a major phase change; that is, the major constituent form isAl₁₈ B₄ O₃₃. The material may be further heated to 1300° C. However,this can detrimentally affect the integrity of the fibers in thecomposite. Calcination of alumina and boria powder at a temperature ofabove 1035° C. results in the formation of Al₁₈ BO₄ O₃₃.

The monolithic Al₁₈ B₄ O₃₃ material, when sintered to a high density,e.g., 2.92 gm/cc (99% dense), has a thermal expansion coefficient of4×10⁻⁶ cm/cm°K., and thermal conductivity of 0.065 W/cm/°K. In addition,the material had a MOR strength of 30-35,000 psi and a hardness of1100-1260 VPN.

When an organic binder is used, the binder solution may containmonomers, plasticizer and a solvent. The solution typically isapproximately 15% of the entire slurry, when the powder density isapproximately 3-4 g/cm³. This organic binder is described in WO88/07505, incorporated herein by reference, parts of which arereproduced below for convenience. Using the binder, even though there isa high solids content, the slurry can be made to infiltrate and wet thefibers. Typically, the viscosity ranges from 50 to 50,000 with 50 to5,000 cps being preferred. However, it is to be understood that thepreferred viscosity will depend on the fabrication equipment, the shapeof the piece being formed and the fiber web or mat to be infiltrated. Itis important that the slurry can flow readily without the need to usehigh pressure to induce flow or to overcome a yield stress to flow. Thepresent invention can provide slurry viscosities of less than 1000 cps(at 100 s⁻¹) at solids loading of at least about 50 vol. %. Suchslurries are not only pourable, but are injectable under extremely low,virtually zero pressure (i.e., gauge pressure as opposed to absolute).

The monomers used in the dispersant solution can be any vinyl or acrylicmonomer or mixtures thereof, or they may also include oligomers withvinyl (e.g., acrylic) functionalities. They may also be multi-functionalor can contain other reactive moieties, such as hydroxyl, epoxy, orurethane groups. In preferred embodiments, the monomer may be chosenfrom among the acrylates, styrenes, vinyl pyridines, other vinylcompounds, or a mixture of these or their derivatives. Afterpolymerization, these yield polyacrylates, polystyrenes, poly (vinylpyridines), polyvinyls, or a mixture of these polymers or copolymers ortheir derivatives. The term "monomer" is used herein to connote bothmonomers and oligomers, components that are essentially unpolymerizedwith respect to the final polymerization product.

The monomers may make up about 50 wt. % of the binder solution, or about7 wt. % to about 10 wt. % of the entire slurry, but higher or lowerlevels of monomers may be useful depending on the shape of the piecebeing formed.

The remaining portion of the binder solution (or about 7%-10% by weightof the slurry) is made up of various volatile organic solvents. Thisadditional component of the dispersant solution may containplasticizers, diluents, and dispersants. Typical plasticizers aredibutylphthalate and other phthalate esters. Examples of diluentsinclude decalin and volatile fatty acids or esters, such as oleic acid.Commonly used dispersants include GAFAC RE-610 (an anionicpolyoxyethylene nonylphenyl ether phosphate, available from the GAFCorp., Wayne, N.J.), AEROSOL OT (a dioctyl ester of sodium sulfosuccinicacid, available from American Cyanamid, Danbury, Conn.), organictitanates such as KR TTS or KR-7 (both available from KenrichPetrochemicals, Inc., Bayonne, N.J.), SPAN 85 (a nonionic sorbitanmonolaurate, available from ICI Americas, Wilmington, Del.), or EMCOLCC-55 (a cationic polypropoxy quaternary ammonium acetate, availablefrom Witco Chemical Co., Perth Amboy, N.J.). These are important for anumber of reasons. First, they facilitate the ability to achieve apourable slurry which has a high solids loading by decreasingparticle-to-particle interactions (both agglomerative and repulsive).Further, the suspension with the additional organic components canexhibit better isotropic properties in both the green piece and thesintered piece. The particles seem to have a more random orientation,resulting in less internal stress and ultimately fewer defects.

In another embodiment, the in situ polymerization occurs without theneed for elevating the temperature above room temperature (approximately20°-35° C.) although the reaction may occur at elevated temperatures. Inone embodiment, two slurries may be mixed. Both slurries may containceramic particles, monomers, and organic solvents. One slurry maycontain a catalyzable initiator, such as benzoyl peroxide, and thesecond contains a compound which catalyzes the initiator. Examples ofsuch catalysts can be found in U.S. Pat. Nos. 3,991,008 and 3,591,438.Preferred compounds of this type include dimethyl aniline, dimethyltoluidine, and thioureas. As the two slurries are mixed, the reaction ofthe catalyst with the initiator triggers in situ polymerization.Alternatively, the catalyst may, of course, be added directly to aslurry containing the initiator. Yet another alternative, depending onthe shape produced, is to photoinitiate polymerization.

Types of ceramic fibers which can be used in the matrix include siliconcarbide, alumina, alumina-boria-silica, silica, titanium boride, boron,carbon, aluminum silicate and aluminum nitride.

Silicon carbide fibers in the form of a web which has been found toquite suitable are available from Dow Corning, Midland, Mich., under thename Nicalon. Also, a web of alumina-boria-silica fibers has been foundto be suitable and is available from 3M Corporation, St. Paul, Minn.,under the name Nextel. The fibers may be coated to avoid reaction of thefiber surface with the ceramic constituent during sintering, if desired.It has been found that interlocking of the ceramic and fiber as byreactions may, in certain instances, be detrimental to the performanceof the ceramic matrix composite. Thus, for example, the silicon carbidefibers of the Nicalon web may be coated with, for example, boron nitrideapplied over pyrolytic carbon coated ceramic fiber cloth to prevent orcontrol any reaction that might occur with the fibers during sinteringto form the aluminum borate matrix. That is, the coating can maintain astable interface between the fiber and the matrix. Pyrolytic carbon, BN,TiN and TiC coating may also be useful.

The volume percent of fibers in the sintered body can range from 20 to60%, preferably 25 to 50% with typical amounts being about 28 to 35%.

After the Al₂ O₃ /B₂ O₃ powder slurry has been made, it is introduced toa body of fibers, e.g., web or mat in a manner that lets it permeatethrough the fibers of the web or bat, sometimes referred to as slurryinfiltration. Upon reaching the desired loading of the web with slurry,the infiltrated webs may be layered to provide several webs of fiberthroughout the composite body. It will be understood that whilereference is made here to flat bodies such as tiles, otherconfigurations, such as pipe shapes or more complex shapes, may beutilized. In the case of pipes, the infiltrated web may be wrappedseveral times, for example. After the shaping or layering, the body canbe pressed or squeezed to remove excess liquid. Thereafter, the body isdried or cured in the range of 40° to 100° C. While curing can occur atroom temperature, the curing can have the effect of polymerizing orcuring the binder faster and driving off the solvents. The solvent mayremain after polymerization and be substantially removed during thebinder burnout stage. Burnout can occur at temperature in the range of300° to 500° F.

It will be appreciated that burnout of the binder can leave voids in thegreen composite. This can lead to an undesirable low density compositeupon sintering. The present invention aids in effectively removing voidsfrom the composite. That is, in reaction sintering, when 9 moles of Al₂O and 2 moles of B₂ O₃ combine to form Al₁₈ B₄ O₃₃, for example, thereis an increase of 14% in volume when all the reactants are converted toAl₁₈ B₄ O₃₃. It will be appreciated that if only half of the reactantsare converted, the volume expansion will be about 7%, and any partialreaction is contemplated to be within the purview of the invention. Thatis, the volume expansion can be 1 to 14 vol. %. Thus, this increase involume can be used effectively to densify the ceramic matrix composite.Thus, for purposes of the present invention, when it is desired to formAl₁₈ B₄ O₃₃ type compounds, the green composite must be heatedsufficiently. For example, when the green composite is heated to above1035° C., Al₁₈ B₄ O₃₃ is formed. Because aluminum borate expands onforming by sintering, it may be necessary to restrain the greencomposite in a mold to improve on the removal of the voids.

It may be desired to add sintering aids to the slurry. Such aids includeCaO, MgO and CaAl₂ B₂ O₇ which can be in the slurry up to 4 wt. % basedon the solid content. Aids for sintering are disclosed in U.S. Pat. No.4,774,210, incorporated herein by reference.

The ceramic matrix composite has the advantage that contrary to mostceramic bodies, it has a degree of bendability. Further, the compositehas a high level of heat resistance, and thus, has utility in exhaustducts of missles and rockets. Further, because this composite can bemade without metal components, it has low observable properties, andtherefore, finds utility in aircrafts, missles, etc.

EXAMPLE 1

A powder mixture containing 9 moles of Al₂ O₃ and 2 moles of B₂ O₃ and1% CaO was ball milled with alumina balls. This material was used tomake a slurry containing 57 vol. % solids with a liquid containing 42.3wt. % n-butyl methacrylate, 2.4 wt. % methacrylic acid, 2.4 wt. %diethylene glycol dimethacrylate, 29.4 wt. % dibutyl phthalate, 14.7 wt.% decalin and 8.8 wt. % RE610. To the slurry was added 1 wt. % benzoylperoxide as initiator. A plain weave Nicalon cloth, heat cleaned toremove sizing, was used for composite lay-up. Nine layers of 4"×6"Nicalon cloth was used to make a slurry infiltrated composite panel. Theslurry was poured onto one layer of cloth placed over aluminum foil. Theslurry was spread and allowed to infiltrate the fiber tows of the cloth.Air bubbles were removed by using a steel roller. Another layer of clothwas placed on the first layer and slurry again poured on top of thecloth. This method was repeated for 9 layers. Excess liquid was squeezedout using pressure of 500 psi. At the same time, the part was cured at65° C. for 1 hour. The binder was burned out in an argon atmospherestarting at 25° C. and ending at 400° C., the temperature being raisedover a period of 371/2 hours and held at 400° C. for 1 hour. Afterburnout, the body was sintered at 1080° C. for 2 hours.

The sintered body contained 29.9 vol. % fiber and the matrix was 60%dense when compared to theoretical density of the matrix. The body had aflexure strength of 12,000 psi at room temperature. The tensile strengthwas 14,000 psi, and room temperature elongation was 0.7% withnoncatastrophic failure in this sample.

EXAMPLE 2

In this example, the same procedure was used for infiltration, but 8harness satin weave Nicalon cloth with duplex CVD coating of BN overpyrolytic carbon on the cloth fibers was used. Approximately 55 vol. %solid was present in the slurry. Sintered part contained 29.4 vol. %fiber, and density of the matrix was 58% of theoretical. The part showeda flexure strength of 28,000 psi after one hour heat treatment at 1500°F. in air. This shows that the part has elevated temperature capability.

FIGS. 1 and 2 show a fracture surface of Al₁₈ B₄ O₃₃ -NiCAlON compositeas produced in Example 2, and there is shown fiber pullout andnoncatastrophic failure.

Having thus described the invention, what is claimed is:
 1. A method offorming an improved ceramic matrix composite resulting from formation ofaluminum borate from alumina and boria comprising the steps of:(a)providing a slurry comprised of aluminum oxide, boron oxide and anorganic binder suspended in a liquid, the aluminum oxide and boron oxidebeing present to form aluminum borate having the formula Al_(x) B_(y)O_(z) where x is in the range of 16 to 20, y is in the range of 3 to 5and z is in the range of 30 to 36; (b) introducing said slurry to a bodyof ceramic fibers to provide a green composite, the body of ceramicfibers containing at least one of silicon carbide, alumina,alumina-boria-silica, titanium boride, boron, carbon, aluminum silicateand aluminum nitride fibers; (c) removing said liquid and said organicbinder from said green composite; and (d) sintering said green compositeat a temperature of greater than 800° C. to cause a reaction of aluminumoxide and boron oxide to form a ceramic matrix composite comprised ofsaid body of ceramic fibers and aluminum borate, the reaction providinga volume expansion which aids in removal of pores resulting from saidorganic binder removal.
 2. The method in accordance with claim 1 whereinthe fibers are provided as a web which includes silicon carbide fibers.3. The method in accordance with claim 1 wherein the fibers are providedas a web which includes alumina fibers.
 4. The method in accordance withclaim 1 wherein the fibers are provided as a web which includesalumina-boria-silica fibers.
 5. The method in accordance with claim 1wherein the fibers are provided as a web which includes silica fibers.6. The method in accordance with claim 1 wherein the fibers are providedas a web which includes titanium boride fibers.
 7. The method inaccordance with claim 1 wherein the fibers are provided as a web whichincludes carbon fibers.
 8. The method in accordance with claim 1 whereinthe fibers are provided as a web which includes aluminum nitride fibers.9. The method in accordance with claim 1 wherein the slurry containsalumina to boria in a mole ratio in the range of 2 to
 8. 10. The methodin accordance with claim 1 wherein the slurry contains about 9 moles ofalumina and about 2 moles of boria.
 11. The method in accordance withclaim 1 wherein the slurry contains 40 to 70 vol. % solids including Al₂O₃ and B₂ O₃.
 12. The method in accordance with claim 1 wherein theceramic matrix composite contains 20 to 60 vol. % fibers.
 13. The methodin accordance with claim 1 wherein the ceramic matrix composite contains25 to 50 vol. % fibers.
 14. The method in accordance with claim 1wherein the ceramic matrix composite contains 28 to 35 vol. % fibers.15. The method in accordance with claim 1 wherein the slurry contains asintering aid.
 16. The method in accordance with claim 15 wherein thesintering aid is selected from CaO, MgO and CaAl₂ B₂ O₇.
 17. The methodin accordance with claim 15 wherein the slurry contains up to 4%sintering aid.
 18. The method in accordance with claim 1 wherein thematrix is Al₁₈ B₄ O₃₃.
 19. The method in accordance with claim 1 whereinthe body is a web.
 20. The method in accordance with claim 1 wherein theorganic binder is burned out.
 21. The method in accordance with claim 1wherein the sintering is performed in a temperature range of 800° to1200° C.
 22. A method of forming an improved ceramic matrix compositeusing the expansion effect resulting from formation of aluminum boratefrom alumina and boria comprising the steps of:(a) providing a slurrycomprised of alumina and boria in the ratio of 9 moles of alumina to 2moles of boria and an organic binder solution containing a monomer and aliquid solvent, the slurry comprising 40 to 70 vol. % alumina and boria,the remainder the organic binder solution, the alumina and boria capableof reacting to form Al₁₈ B₄ O₃₃ ; (b) introducing said slurry to a webof fibers selected from silicon carbide, alumina-boria and carbon fibersto provide an infiltrated web; (c) removing liquid solvent from saidgreen composite; (d) removing said organic binder by heating; and (e)sintering said green composite at from 800° to 1300° C. to cause areaction of alumina and boria to form a ceramic matrix compositecomprised of said web of fibers and aluminum borate having the formulaAl₁₈ B₄ O₃₃, the reaction providing up to a 14% volume expansion in thegreen composite which aids in filling pores resulting from the organicbinder removal.
 23. A method of forming an improved ceramic matrixcomposite using the expansion effect resulting from formation ofaluminum borate from alumina and boria comprising the steps of:(a)providing a slurry comprised of alumina and boria in the ratio of 9moles of alumina to 2 moles of boria and an organic binder solutioncontaining a monomer and a liquid solvent, the slurry comprising 40 to70 vol. % alumina and boria, the remainder the organic binder solution,the alumina and boria capable of reacting to form aluminum borate; (b)introducing said slurry to a web of fibers selected from siliconcarbide, alumina-boria and carbon fibers to provide an infiltrated web;(c) removing liquid solvent from said green composite; (d) removing saidorganic binder by heating; and (e) sintering said green composite atfrom 800° to 1300° C. to cause a reaction of alumina and boria to form aceramic matrix composite comprised of said web of fibers and aluminumborate having the formula Al_(x) B_(y) O_(z) wherein x is equal to16-20, y is equal to 3-5 and z is equal to 30-36, the compositecontaining 20 to 60 vol. % fiber, the remainder aluminum borate, thereaction causing a volume expansion in said green composite whicheffectively fills voids in said composite resulting from the organicbinder removal.